Average protein density is a molecular-weight-dependent function.

Abstract

The mass density of proteins is a relevant basic biophysical quantity. It is also a useful input parameter, for example, for three-dimensional structure determination by protein crystallography and studies of protein oligomers in solution by analytic ultracentrifugation. We have performed a critical analysis of published, theoretical, and experimental investigations about this issue and concluded that the average density of proteins is not a constant as often assumed. For proteins with a molecular weight below 20 kDa, the average density exhibits a positive deviation that increases for decreasing molecular weight. A simple molecular-weight-depending function is proposed that provides a more accurate estimate of the average protein density.

Square symbols correspond to the densities of a number of proteins calculated by . The solid line is the best fit of an exponential function (equation 1) to those values. Dotted lines are the limits associated to ±1 standard deviation (σ = 0.014 g/cm3). (Inset) An enlarged view of the low molecular weight part of the curve.

Experimental (filled circles) and theoretical (open squares) densities determined by for a number of proteins. The solid line (above) and dashed line (below) correspond to the best fits of equation 1 to the calculated and experimental densities, respectively.